Deposition systems and methods are commonly used to form layers of semiconductor materials, such as thin epitaxial films (epilayers), on substrates. For example, a chemical vapor deposition (CVD) reactor system and process may be used to form a layer of semiconductor material such as silicon carbide (SiC), gallium nitride (GaN), or other materials on a substrate. CVD processes may be particularly effective for forming layers with controlled properties, thicknesses, and/or arrangements such as epitaxial layers. Typically, in a deposition system, such as a CVD system, the substrate is placed in a reaction chamber within a susceptor and one or more process gases including reagents or reactants to be deposited on the substrate are introduced into the chamber adjacent the substrate. The process gases may be flowed through the reaction chamber in order to provide a uniform or controlled concentration of the reagents or reactants to the substrate.
CVD growth processes for silicon carbide have been refined in terms of temperature profiles, gas velocities, gas concentrations, chemistry, and pressure. The selection of conditions used to produce particular epilayers is often a compromise among factors such as desired growth rate, reaction temperature, cycle time, gas volume, equipment cost, doping uniformity, and layer thicknesses. In particular, and other factors being equal, uniform layer thicknesses tend to provide more consistent performance in semiconductor devices that are subsequently produced from the epitaxial layers. Alternatively, less uniform layers tend to degrade device performance, or even render the layers unsuitable for device manufacture. Accordingly, a need exists for CVD techniques for the epitaxial growth of, for example, silicon carbide, gallium nitride, or other materials that produce more uniform epitaxial layers.